High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al., U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
A useful family of such compositions are those which are glass-reinforced. For instance, Zimmerman, U.S. Pat. No. 3,814,725 discloses that improved molding resins are obtained by blending polyesters such as polypropylene terephthalate and polybutylene terephthalate with glass fibers.
It is known that when glass fibers having a standard diameter of 0.00051 to 0.00055 inches are used in various thermoplastic polyester formulations where the glass fiber concentration is less than 20%, the deflection temperature under load, as measured by the ASTM D-648 test, at 264 psi, is likely to be substantially less than 415.degree. F., a value for typical polyester formulations having 30% glass by weight. Moreover, as the glass fiber concentration is reduced further below 20%, the heat deflection temperature is still further reduced. The heat deflection temperature is also reduced if a glass reinforced polyester is extruded more than once, as is the case where scrap material is reused by regrinding and then remolding or re-extruded into pellets for subsequent extrusion or molding. This reduction in the heat deflection temperature under load is undesirable when the thermoplastic polyester is to be used in applications involving exposure to elevated temperatures. At the same time, however, it is desirable to keep the glass fiber concentration low, in order to reduce the tendency of molded parts comprising glass reinforced polyesters to warp.
It has now been surprisingly discovered that by using glass fibers having a smaller average diameter, i.e., about 0.0005 inches or less, the heat deflection temperature is reduced to a much lesser extent. For example, in two polyester formulations containing, respectively, 7 and 11% by weight of glass fibers having an average diameter of 0.00037 inches, the deflection temperature under load, as measured by ASTM D-648, is 16.degree. F. and 7.degree. F. higher than control formulations containing glass filaments of a larger diameter. Even more significantly, after four re-extrusions (five extrusions total) prior to molding, the heat deflection of the subsequently molded parts was 37.degree. and 28.degree. F., respectively, greater than the larger diameter control. The extent of the difference is surprising in view of the relatively small change in glass fiber diameter.